1. Field of the Invention
This invention pertains to the general field of optical profilometry and, in particular, to interferometric measurements conducted through a transmissive medium.
2. Description of the Prior Art
Interferometric optical systems are widely used to measure surface features because of their speed, accuracy and flexibility. As is well understood in the art, conventional phase shifting interferometry (PSI) requires that the surface of the test sample being profiled be quite smooth, so that continuous interference fringes are obtained and so-called “phase ambiguities” are avoided. Various kinds of “phase unwrapping” algorithms are used to track phase over a large range of surface heights in order to resolve phase-ambiguity errors. With single-wavelength techniques, the maximum surface roughness is limited to approximately 1/100th the wavelength of the light used and step-height measurements are limited to steps no greater than approximately one eight the wavelength. Vertical scanning interferometry (VSI), in which white-light interference fringes are demodulated to find the peak amplitude of an envelope of the fringes to determine the height value at the peak interference fringe, is preferably used to produce area profiles of rough surfaces with height variations exceeding the measurement range of single-wavelength techniques. VSI provides the advantage of producing rapid results without the need for phase-unwrapping algorithms.
Interferometric profilers based on these techniques are used in particular to measure micro-electro-mechanical systems (MEMS). MEMS integrate mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. By combining silicon-based microelectronics and micromachining technology in MEMS, it is now possible to realize complete on-a-chip systems that promise to revolutionize nearly every product category.
MEMS devices are manufactured using batch fabrication techniques similar to those used for integrated circuits and quality control is a key to making a successful product. Since approximately 80% of the total cost of MEMS production resides in packaging and testing, accurate metrology of the devices is essential for commercial success. Moreover, the devices need to be tested in their final packaged state, typically underneath a protective surface such as glass, plastic, or sapphire, which is an essential component of the packaged product. These transmissive fixed media often degrade the interferometric measurement because of dispersion and aberration effects. For example, the top portion of FIG. 1 shows the clear image of a pitch standard on a mirror surface produced by a conventional interferometric profiler and the top portion of the figure illustrates the high-contrast fringes produced by tilt in the mirror. By contrast, FIG. 2 illustrates the barely visible image of the pitch standard and the interferometric fringes produced by the same profiler after the introduction of a transmissive layer in the optical path of the test beam. Clearly, the image and the corresponding fringes acquired in conventional manner are not suitable for meaningful analysis of a sample that includes a transparent window. Moreover, longer working-distance optics are required to accommodate the additional distance to the test surface when such a protective layer and a beam splitter are present in the sample.
In addition to packaged MEMS devices, other parts requiring precision metrology also warrant the ability to obtain data through dispersive media. These include parts placed in an environmental chamber to study the effects of pressure, temperature, humidity and reactants on the sample. Also, some parts are used while immersed in a liquid medium, including objects used in biology and ink-jet printing, and hard-drive sliders which fly through a lubricating medium. Performing high lateral-resolution metrology on such parts cannot be accomplished using conventional methods.
Thus, when such a transmissive fixed layer is present in the path of the measurement beam of an interferometric profiler, an equivalent compensation element in the path of the reference beam has been used in the past to minimize the dispersion, aberration and interference effects of the transmissive layer. While this solution is normally acceptable for low magnification systems (less than about 10×), it has been found to be unacceptable at higher magnifications where all systems defects tend to become more and more significant. Even the use of an objective specifically corrected for the aberrations introduced by the transmissive layer was surprisingly found not to improve significantly the quality of the fringes produced by conventional interferometric profilers. Therefore, existing interferometric techniques need to be improved to measure sample surfaces lying underneath transmissive fixed materials. The present invention provides various solutions to that end.